High-speed differential comparator

ABSTRACT

A high-speed differential comparator is disclosed. The comparator (100) includes a transconductance device (102 through 112) that receives first and second input voltages (V IN  and -V IN ) and generates first and second currents in response to the first and second input voltages. A first resonant tunneling diode (118) conducts the first current and generates a first output voltage (V OUT1 ) at a first output terminal (105) in response to the first current. A second resonant tunneling diode (126) conducts the second current and generates a second output voltage (V OUT2 ) at a second output terminal (109) in response to the second current. The comparator responds to input voltages at high speed and may be used for high frequency signal sampling and level determination.

RELATED APPLICATIONS

This application is related to a provisional application entitled "High-Speed Differential Comparator," filed Jan. 6, 1998, having a Ser. No. 60/070,553.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of analog signal processing and more particularly to a high-speed differential comparator.

BACKGROUND OF THE INVENTION

In the field of analog signal processing, the use of comparators to determine a signal level is well known. Typically, such comparators have been limited in their response time due to the circuit components making up the comparators. As a result, high-speed and/or high-frequency voltage level comparison has not been possible.

SUMMARY OF THE INVENTION

Therefore, a need has arisen for a high-speed comparator that addresses the disadvantages and deficiencies of the prior art.

A high-speed differential comparator is disclosed. In one embodiment, the comparator comprises a transconductance device that receives first and second input voltages and generates first and second currents in response to the first and second input voltages. A first resonant tunneling diode conducts the first current and generates a first output voltage at a first output terminal in response to the first current. A second resonant tunneling diode conducts the second current and generates a second output voltage at a second output terminal in response to the second current.

A technical advantage of the present invention is that the comparator responds to input voltages at high speed. Another technical advantage is that a signal level determination circuit utilizing the comparator may be used for high frequency signal sampling.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding Of the present invention and for further features and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a differential comparator circuit constructed in accordance with one embodiment of the present invention;

FIG. 2 is a graph of voltage versus current for an exemplary resonant tunneling diode;

FIG. 3 is a graph illustrating the output characteristics of the differential comparator circuit;

FIG. 4 is a schematic diagram of a symmetrical resonant tunneling diode arrangement that improves the symmetry of the differential comparator circuit;

FIG. 5 is a schematic diagram in partial block form of a signal level determination circuit utilizing the differential comparator circuit;

FIGS. 6 and 7 are graphs illustrating the output of the signal level determination circuit;

FIG. 8 is a schematic diagram of a differential comparator circuit constructed in accordance with another embodiment of the present invention;

FIG. 9 is a schematic diagram of a differential comparator circuit constructed in accordance with yet another embodiment of the present invention,

FIG. 10 is an alternative load configuration that may be used with the differential comparator circuit;

FIG. 11 is another alternative load configuration that may be used with the differential comparator circuit; and

FIG. 12 is yet another alternative load configuration that may be used with the differential comparator circuit.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a differential comparator circuit 100 is shown. In this circuit, an input voltage V_(IN) is received at the gate of a first field effect transistor (FET) 102. The inverse of V_(IN) (-V_(IN)) is supplied to the gate of a second FET 104. Alternatively, a reference voltage V_(REF) may be supplied to the gate of FET 104.

FET 102 has a drain connected to an output node 105 and a source connected to node 106. A current sink 108 is connected between node 106 and ground. FET 104 has a drain connected to a second output node 109 and a source connected to node 110. A current sink 112 matched to current sink 108 is connected between node 110 and ground. Current sinks 108 and 112 each sink a current I. A resonant tunneling diode 114 is connected between nodes 106 and 110.

A resonant tunneling diode 118 is connected between a voltage supply V_(DD) and output node 105. A reset FET 120 is connected between V_(DD) and output node 105, in parallel with RTD 118. Reset FET 120 receives a RESET signal at its gate to reset the output voltage V_(OUT1) at output node 105.

Likewise, a resonant tunneling diode 126 is connected between V_(DD) and output node 109. A second reset FET 128 is connected between V_(DD) and output node 109, in parallel with RTD 126. Reset FET 128 receives the RESET signal at its gate to reset the output voltage V_(OUT2) at output node 109.

In operation, differential comparator circuit 100 alternates between a comparison cycle and a reset cycle. During the comparison cycle, FETs 102 and 104 conduct a combined current of 2I. When the input voltages supplied to the gates of FETs 102 and 104 are equal, FETs 102 and 104 each conduct a current I equal to the current conducted by each current sink 108 and 112. If the voltage supplied to the gate of FET 104 is -V_(IN), then this occurs when V_(IN) =-V_(IN) =0. If the voltage supplied to the gate of FET 104 is V_(REF), then this occurs when V_(IN) =V_(REF). In either case, this is known as the equilibrium value of V_(IN).

When V_(IN) is above its equilibrium value, FET 102 will conduct a current I+ε and FET 104 will conduct a current I-ε. Conversely, when V_(IN) is more negative than its equilibrium value, FET 102 will conduct a current I-ε and FET 104 will conduct a current I+ε. The value of ε varies from zero to I, depending on the magnitude of the departure of V_(IN) from its equilibrium value. A current equal to ε is conducted by RTD 114 to compensate for the difference in current conducted by FETs 102 and 104. RTD 114 functions merely as a resistance between nodes 106 and 110, and does not utilize the characteristics unique to resonant tunneling diodes. RTD 114 may therefore be replaced by a resistor.

During the comparison cycle, the RESET signal is low, and FETs 120 and 128 act as open switches. Thus, the current conducted by FET 102 is also conducted by RTD 118. Likewise, the current conducted by FET 104 is also conducted by RTD 126.

The voltage drop V_(RTD1) or V_(RTD2) across each respective RTD 118 and 126 varies with the current conducted by the RTD as shown in FIG. 2. Thus, when the input voltage V_(IN) is at its equilibrium value, each RTD conducts an equilibrium current I₀ which creates a voltage drop V₀ across the RTD. When the RTD current reaches a transition value I₁, the voltage drop across the RTD transitions discontinuously from V₁ to V₂.

FIG. 2 shows the current-voltage profile only for a forward-biased RTD. However, it will be understood in the following description that where an RTD may be biased in either direction, as with RTD 114, a symmetric or nearly symmetric RTD is preferably used. Alternatively, a symmetric RTD arrangement such as that illustrated in FIG. 4 may be used, as will be explained more fully below. For RTDs which are only biased in one direction, such as RTDs 118 and 126, symmetry need not be a consideration.

The voltage drop across RTD 118 results in an output voltage V_(OUT1) =V_(DD) -V_(RTD1) at output node 105. Thus, for example, if V_(IN) >-V_(IN) and I+ε<I₁, then V_(RTD1) <V₁ and V_(OUT1) >V_(DD) -V₁. However, if V_(IN) >-V_(IN) and I+ε>I₁, then V_(RTD1) >V₂ and V_(OUT1) <V_(DD) -V₂. In other words, when the input voltage V_(IN) crosses a certain positive threshold value (V_(T+)), the first output voltage V_(OUT1) transitions discontinuously from V_(DD) -V₁ to V_(DD) -V₂, while the second output voltage V_(OUT2) remains above V_(DD) -V₁.

Likewise, if V_(IN) <-V_(IN) and I+ε<I₁, then V_(RTD2) <V₁ and V_(OUT2) >V_(DD) -V₁. However, if V_(IN) <-V_(IN) and I+ε>I₁, then V_(RTD2) >V₂ and V_(OUT2) <V_(DD) -V₂. In other words, when the input voltage V_(IN) reaches a certain negative threshold value (V_(T-)), the second output voltage V_(OUT2) transitions discontinuously from V_(DD) -V₁ to V_(DD) -V₂, while the first output voltage V_(0UT2) remains above V_(DD) -V₁.

These exemplary output characteristics of differential comparator circuit 100 are illustrated in FIG. 3. It will be recognized that if differential comparator circuit 100 is completely symmetrical, then V_(T+) =-V_(T-). It will also be recognized that, in the alternative embodiment in which the gate voltage of FET 104 is kept at a reference voltage VEF rather than -V_(IN), similar output characteristics will be observed, but with the horizontal axes of FIG. 3 being centered on V_(REF) rather than zero.

Differential comparator circuit 100 therefore acts as a threshold detector with three valid output states:

(1) If V_(OUT1) =HIGH and V_(OUT2) =HIGH, then V_(T-) <V_(IN) <V_(T+).

(2) If V_(OUT) =LOW and V_(OUT2) =HIGH, then V_(IN) >V_(T+).

(3) If V_(OUT) =HIGH and V_(OUT2) =LOW, then V_(IN) <V_(T-).

In these output states HIGH represents a voltage greater than or equal to V_(DD) -V₁, whereas LOW represents a voltage less than or equal to V_(DD) -V₂.

During the reset cycle of differential comparator circuit 100, the RESET signal is high. FE.s 120 and 128 therefore act as closed switches, shorting the output nodes 105 and 109 to V_(DD). Periodic resetting of differential comparator circuit 100 is beneficial because of the hysteresis introduced into differential comparator circuit 100 by RTDs 118 and 126. If either output voltage V_(OUT1) or V_(OUT2) assumes a LOW value, then the aforementioned output states of differential comparator circuit 100 may not properly indicate the value of the input voltage until the circuit is reset. Thus, the RESET signal may be, for example, a clock signal with a frequency of up to 25 GHz or even higher.

Referring to FIG. 4, a symmetrical RTD arrangement 140 that may be used to improve the symmetry of differential comparator circuit 100 is shown. It is well known that a resonant tunneling diode exhibits a slight asymmetry in its V-I curve, depending on the direction in which current is flowing through the RTD. In symmetrical RTD arrangement 140, two resonant tunneling diode 142 and 144 are connected in parallel between first and second nodes 146 and 148. RTD 142 has a first side 150 connected to node 146 and a second side 152 connected to node 148. First side 150 represents the substrate side of RTD 142. Likewise, RTD 144 has a first side 154, representing the substrate side of RTD 144, connected to node 148. A second side 156 is connected to node 146. Symmetrical RTD arrangement 140 exhibits a symmetrical V-I response curve. Symmetrical RTD arrangement 140 may be substituted for RTD 114 in differential comparator circuit 100 to improve the output symmetry of the circuit.

Referring to FIG. 5, a signal level determination circuit 150 utilizing differential comparator circuit 100 is shown. Signal level determination circuit 150 includes a set of primary differential comparators such as comparators 152-154. Each primary differential comparator 152-154 receives an input voltage (V_(IN)) and its inverse (-V_(IN)) and generates an output at two output terminals. Each primary differential comparator may comprise a differential comparator circuit such as differential comparator circuit 100, or any other embodiment of the present invention described below. Each primary differential comparator receives a RESET signal (not shown), which is preferably a clock signal with a frequency equal to the desired sampling frequency for input voltage V_(IN).

For illustrative purposes, only three primary differential comparators are shown. However, it will be understood that any number of primary differential comparators may be included in signal level determination circuit 150.

The various outputs of primary differential comparators 152-154 are received by a set of secondary differential comparators 161-164 in a manner described more fully below. Each secondary differential comparator also receives the same RESET signal received by primary differential comparators 152-154. Each secondary differential comparator generates two outputs in response to the inputs.

Again, the number of secondary differential comparators shown is for illustrative purposes only, and it will be understood that any number of secondary differential comparators may be included in signal level determination circuit 150. Generally, the number of secondary differential comparators will be approximately equal to the number of primary differential comparators, in accordance with the connection scheme described below.

Taking primary differential comparator 153 as an example, this comparator has a first output V_(OUT3) which is supplied to a first input of secondary differential comparator 163 and a second input of secondary differential comparator 161. Primary differential comparator 153 has a second output V_(OUT4) which is supplied to a first input of secondary differential comparator 164 and a second input of secondary differential comparator 162.

This pattern continues throughout signal level determination circuit 150. Thus, each primary differential comparator N has a first output supplied to a first input of a corresponding secondary differential comparator n. The first output of primary differential comparator N is also supplied to a second input of secondary differential comparator n-2. The second output of primary differential comparator N is supplied to a first input of secondary differential comparator n+1 and a second input of secondary differential comparator n-1.

In this arrangement, each secondary differential comparator n, when n is odd, receives a first input from the first output of primary differential comparator N and receives a second input from the first output of primary differential comparator N+1. When n is even, secondary differential comparator n receives a first input from the second output of primary differential comparator N and a second input from the second output of primary differential comparator N+1.

The output generated by signal level determination circuit 150 is shown in FIGS. 6 and 7. Referring first to FIG. 6, when the input voltage V_(IN) rises above a threshold voltage V_(T1+), the first output voltage V_(OUT1) of primary differential comparator 152 changes from HIGH to LOW, while the second output voltage V_(OUT2) remains HIGH. When the input voltage V_(IN) drops below a the inverse threshold voltage V_(T1-), the second output voltage V_(OUT2) of primary differential comparator 152 changes from HIGH to LOW, while the first output voltage V_(OUT1) remains HIGH.

Likewise, primary differential comparators 153 and 154 each have their own positive threshold voltages V_(T2+) and V_(T3+), respectively, and their own negative threshold voltages V_(T2-) and V_(T3-), respectively. The components of primary differential comparators 152-154 may be adjusted so that the respective threshold voltages are separated by regular intervals such as, for example, 100 millivolts. This separation interval determines the resolution of signal level determination circuit 150, as will become apparent from the description below. Thus, signal level determination circuit 150 should have primary differential comparators with threshold voltages evenly spaced throughout the desired detection range of input voltage V_(IN).

Referring to FIG. 7, the output of secondary differential comparators 161-164 is shown. Taking secondary differential comparator 163 as an example, this comparator receives as its input the output voltage V_(OUT3) from primary differential comparator 153 and the output voltage V_(OUT5) from primary differential comparator 154.

When the input voltage V_(IN) is less than V_(T3+), both V_(OUT3) and V_(OUT5) are HIGH. With its two inputs at approximately the same level, secondary differential comparator 163 is at or near equilibrium, and both its outputs V_(OUT11) and V_(OUT12) are HIGH.

When the input voltage V_(IN) is between V_(T3+) and V_(T2+), V_(OUT3) remains HIGH while V_(OUT5) is LOW. This disparity in inputs causes V_(OUT1) to become LOW, while V_(OUT12) remains HIGH.

When the input voltage V_(IN) is greater than V_(T2+), both V_(OUT3) and V_(OUT5) are LOW. With its two inputs at approximately the same level, secondary differential comparator 163 is at or near equilibrium, and both its outputs V_(OUT11) and V_(OUT12) are HIGH.

Thus, output voltage V_(OUT11) indicates whether the input voltage V_(IN) is between V_(T2+) and V_(T3+). Likewise, output voltage V_(OUT7) from secondary differential comparator 161 indicates whether the input voltage V_(IN) is between V_(T1+) and V_(T2+).

As another example, secondary differential comparator 162 receives as its input the output voltage V_(OUT2) from primary differential comparator 152 and the output voltage V_(OUT4) from primary differential comparator 153. Referring to FIG. 6, it may be seen that these inputs are disparate when the input voltage V_(IN) is between V_(T1-) and V_(T2-). Thus, in this input voltage range, output voltage V_(OUT9) is LOW. For all other input voltage values, V_(OUT9) is HIGH.

Thus, output voltage V_(OUT9) indicates whether the input voltage V_(IN) is between V_(T1-) and V_(T2-). Likewise, output voltage V_(OUT13) from secondary differential comparator 164 indicates whether the input voltage V_(IN) is between V_(T2-) and V_(T3-).

The outputs of secondary differential comparators 161-164 may therefore be used to determine the magnitude and polarity of input voltage V_(IN). With enough primary and secondary differential comparators, signal level determination circuit 150 may be used to detect the input voltage level with any desired resolution. Moreover, because each differential comparator in signal level determination circuit 150 responds to its input at high speed, the circuit may provide signal level detection with a high sampling frequency. The sampling frequency of a signal level determination circuit 150 is determined by the frequency of the RESET signal used in the circuit. Given the short response time of the individual comparators, the sampling frequency may be as high as 25 GHz or even higher.

Referring to FIG. 8, a second differential comparator circuit 200 similar to differential comparator circuit 100 is shown. An input voltage V_(IN) is received at the gate of a first FET 202. The inverse of V_(IN) (-V_(IN)) is supplied to the gate of a second FET 204. Alternatively, a reference voltage V_(REF) may be supplied to the gate of FET 204.

FET 202 has a drain connected to an output node 205 and a source connected to node 206. A current sink 208 is connected between node 206 and ground. FET 204 has a drain connected to a second output node 209 and a source connected to node 210. A current sink 212 matched to current sink 208 is connected between node 210 and ground. Current sinks 208 and 212 each sink the same current. A resonant tunneling diode 214 is connected between nodes 206 and 210. Alternatively, RTD 214 may be replaced by symmetrical RTD arrangement 140 shown in FIG. 4, as previously described, to improve the symmetry of differential comparator circuit 200.

A first load FET 216 and a resonant tunneling diode 218 are connected in series between a voltage supply V_(DD) and output node 205. A reset FET 220 is connected between V_(DD) and output node 205, in parallel with load FET 216 and RTD 218. Reset FET 220 receives a RESET signal at its gate to reset the output voltage V_(OUT1) at output node 205. A current source 222 supplies a current to output node 205.

Likewise, a second load FET 224 and a resonant tunneling diode 226 are connected in series between V_(DD) and output node 209. A second reset FET 228 is connected between V_(DD) and output node 209, in parallel with load FET 224 and RTD 226. Reset FET 228 receives the RESET signal at its gate to reset the output voltage V_(OUT2) at output node 209. A current source 230 supplies a current to output node 209.

Differential comparator circuit 200 is similar in design to differential comparator circuit 100 and exhibits similar output characteristics. One difference between the two circuits is the presence of current sources 222 and 230 in differential comparator circuit 200. Current sources 222 and 230 preferably supply identical currents to FETs 202 and 204, respectively. This current is preferably less than or equal to the current conducted by current sinks 208 and 212. As a result, each RTD 218 or 226 conducts a current equal to the difference between the current conducted by the respective FET 202 or 204 and the current sourced by the respective current source 222 or 230. Thus, it will be appreciated that the current generated by current sources 222 and 230 determines the threshold voltages V_(T-) and V_(T+) of differential comparator circuit 200.

Another difference between differential comparator circuit 200 and differential comparator circuit 100 is the presence of load FETs 216 and 224. The gate of load FET 216 is connected to output node 209, while the gate of load FET 224 is connected to output node 205. This arrangement produces larger output swings at output nodes 105 and 109.

Referring to FIG. 9, a third differential comparator circuit 300 similar to differential comparator circuits 100 and 200 is shown. In this circuit, an input voltage V_(IN) is received at the gate of a first FET 302. The inverse of V_(IN) is supplied to the gate of a second FET 304. Alternatively, a reference voltage V_(REF) may be supplied to the gate of FET 304.

FET 302 has a drain connected to an output node 305 and a source connected to node 306. A current sink 308 is connected between node 306 and ground. FET 304 has a drain connected to a second output node 309 and a source connected to node 310. A current sink 312 matched to current sink 308 is connected between node 310 and ground. Current sinks 308 and 312 each sink a current I.

A resonant tunneling diode 313 is connected between output nodes 305 and 309. Another resonant tunneling diode 314 is connected between nodes 306 and 310. Alternatively, RTD 314 may be replaced by symmetrical RTD arrangement 140 shown in FIG. 4, as previously described, to improve the symmetry of differential comparator circuit 300.

A reset FET 320 is connected between a voltage supply V_(DD) and output node 305. Likewise, a second reset FET 328 is connected between V_(DD) and output node 309. Reset FETs 320 and 328 receive a RESET signal at their respective gates to reset the output voltages V_(OUT1) and V_(OUT2) at output nodes 305 and 309, respectively.

In operation, differential comparator circuit 300 functions in a manner similar to differential comparator circuits 100 and 200. During a comparison cycle, the RESET signal is low and reset FETs 320 and 328 are off, thus isolating output nodes 305 and 309 from V_(DD). Current sinks 308 and 312 each sink a current I. FETs 302 and 304 each conduct a current I+ε for a total current of 2I. RTD 314 therefore conducts a current ε which is less than or equal to I. Current sources 322 and 330 each supply a current I to output nodes 305 and 309, respectively. Thus, RTD 313 also conducts a current ε.

Referring again to FIG. 2, when the current through RTD 313 reaches a transition value I₁, the voltage drop across RTD 313 (V_(RTD)) transitions discontinuously from V₁ to V₂. The voltage drop across RTD 313 results in a voltage difference between output nodes 305 and 309. Thus, for example, if V_(IN) >-V_(IN) and ε<I₁, then V_(RTD) =V_(OUT2) -V_(OUT1) <V₁. However, if V_(IN) >-V_(IN) and ε>I₁, then V_(RTD) =V_(OUT2) -V_(OUT1) >V₂. In other words, when the input voltage V_(IN) crosses a certain positive threshold value (V_(T+)), the differential output voltage V_(OUT1) -V_(OUT2) transitions discontinuously from -V₁ to -V₂.

Likewise, if V_(IN) <-V_(IN) and ε<I₁, then V_(RTD) =V_(OUT1) -V_(OUT2) <V₁ l. However, if V_(IN) <-V_(IN) and ε>I₁, then V_(RTD) =V_(OUT1) -V_(OUT2) >V₂. In other words, when the input voltage V_(IN) reaches a certain negative threshold value (V_(T-)), the differential output voltage V_(OUT1) -V_(OUT2) transitions discontinuously from V₁ to V₂.

Differential comparator circuit 300 therefore acts as a threshold detector with three output states:

(1) If V_(OUT1) -V_(OUT2) ≧V₂, then V_(IN) <V_(T-).

(2) If V₁ ≧V_(OUT1) -V_(OUT2) ≧-V₁, then V_(T-) <V_(IN) <V_(T+).

(3) If V_(OUT1) -V_(OUT2) ≧-V₂, then V_(IN) >V_(T+).

Referring to FIG. 10, a load configuration 340 that may be substituted for RTD 313 is shown. Load configuration 340 comprises a FET 342 with a drain connected to output node 305 of differential comparator circuit 300. The source of FET 342 is connected to a resonant tunneling diode 344. A second FET 346 has a drain connected to output node 309 of differential comparator circuit 300 and a source connected to RTD 344. The gate of FET 342 is connected to the source of FET 346, and vice versa. This load configuration produces larger output swings than RTD 313 alone can provide.

Referring to FIG. 11, a load configuration 350 that may be substituted for RTD 313 and reset FETs 320 and 328 is shown. In this configuration, reset FETs 320 and 328 are removed, leaving output nodes 305 and 309 decoupled from V_(DD) even during the reset cycle of differential comparator circuit 300. A resonant tunneling diode 352 is connected between output nodes 305 and 309. A reset FET 354 is also connected between output nodes 305 and 309, in parallel with RTD 352. During the reset cycle of differential comparator circuit 300, the RESET signal is high. Reset FET 354 therefore acts as a closed switch, equalizing output nodes 305 and 309.

Referring to FIG. 12, a load configuration 360 combining the aspects of configurations 340 and 350 is shown. Load configuration 360 is connected between output nodes 305 and 309 of differential comparator circuit 300 in place of RTD 313 and reset FETs 320 and 328. The operation of load configuration 360 will be apparent to those skilled in the art in light of the previous descriptions of configurations 340 and 350.

While the invention has been particularly shown and described by the foregoing detailed description, it will be understood by those skilled in the art that various other changes in form and detail may be made without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A comparator comprising:a transconductance device operable to receive first and second input voltages and generate a first current at a first current node and a second current at a second current node in response to the first and second input voltages; a first resonant tunneling diode coupled to the first current node operable to conduct the first current and generate a first output voltage at a first output terminal in response to the first current; a second resonant tunneling diode coupled to the second current node operable to conduct the second current and generate a second output voltage at a second output terminal in response to the second current; and a reset device coupled to the first and second output terminals operable to receive a reset signal and generate a reference voltage at the first and second output terminals in response to the reset signal.
 2. The comparator of claim 1, wherein the second input voltage comprises a reference voltage.
 3. The comparator of claim 1, wherein the second input voltage comprises an inverse of the first input voltage.
 4. The comparator of claim 1, wherein the transconductance device comprises:a current sink; a first transistor having first, second and third terminals, the first terminal being operable to receive the first input voltage, the second terminal being coupled to the current sink, the third terminal being coupled to the first output terminal; and a second transistor having first, second and third terminals, the first terminal being operable to receive the second input voltage, the second terminal being coupled to the current sink, the third terminal being coupled to the second output terminal.
 5. The comparator of claim 4, wherein the transconductance device further comprises:a third transistor having first, second and third, terminals, the first terminal being connected to the second output terminal, the second terminal being connected to a voltage supply, the third terminal being connected to a terminal of the first resonant tunneling diode; and a fourth transistor having first, second and third terminals, the first terminal being connected to the first output terminal, the second terminal being connected to the voltage supply, the third terminal being connected to a terminal of the second resonant tunneling diode.
 6. The comparator of claim 1, wherein the first resonant tunneling diode comprises first and second terminals, the first terminal being coupled to a voltage supply, the second terminal being coupled to the first output terminal; andwherein the second resonant tunneling diode comprises first and second terminals, the first terminal being coupled to the voltage supply, the second terminal being coupled to the second output terminal.
 7. The comparator of claim 1, wherein the reset device comprises:a first reset switch connected between the voltage supply and the first output terminal, the first reset switch being operable to close in response to the reset signal; and a second reset switch connected between the voltage supply and the second output terminal, the second reset switch being operable to close in response to the reset signal.
 8. The comparator of claim 1, further comprising:a first current source coupled to the first output terminal operable to supply a first offset current to the first output terminal, the first offset current offsetting at least a portion of the first current; and a second current source coupled to the second output terminal operable to supply a second offset current to the second output terminal, the second offset current offsetting at least a portion of the second current.
 9. The comparator of claim 4, wherein the current sink comprises:a first current sink coupled to the second terminal of the first transistor; a second current sink coupled to the second terminal of the second transistor; and an impedance connected between the second terminal of the first transistor and the second terminal of the second transistor.
 10. A signal level determination circuit comprising a plurality of comparators, each comparator being operable to receive an input voltage and generate an output voltage in response to the input voltage, at least one of the comparators having:a transconductance device operable to receive first and second input voltages and generate a first current at a first current node and a second current at a second current node in response to the first and second input, voltages; a first resonant tunneling diode coupled to the first current node operable to conduct the first current and generate a first output voltage at a first output terminal in response to the first current; a second resonant tunneling diode coupled to the second current node operable to conduct the second current and generate a second output voltage at a second output terminal in response to the second current; and a reset device coupled to the first and second output terminals operable to receive a reset signal and generate a reference voltage at the first and second output terminals in response to the reset signal.
 11. The signal level determination circuit of claim 10, wherein the second input voltage comprises a reference voltage.
 12. The signal level determination circuit of claim 10, wherein the second input voltage comprises an inverse of the first input voltage.
 13. The signal level determination circuit of claim 10, wherein the transconductance device comprises:a current sink; a first transistor having first, second and third terminals, the first terminal being operable to receive the first input voltage, the second terminal being coupled to the current sink, the third terminal being coupled to the first output terminal; and a second transistor having first, second and third terminals, the first terminal being operable to receive the second input voltage, the second terminal being coupled to the current sink, the third terminal being coupled to the second output terminal.
 14. The signal level determination circuit of claim 13, wherein the transconductance device further comprises:a third transistor having first, second and third terminals, the first terminal being connected to the second output terminal, the second terminal being connected to a voltage supply, the third terminal being connected to a terminal of the first resonant tunneling diode; and a fourth transistor having first, second and third terminals, the first terminal being connected to the first output terminal, the second terminal being connected to the voltage supply, the third terminal being connected to a terminal of the second resonant tunneling diode.
 15. The signal level determination circuit of claim 10, wherein the first resonant tunneling diode comprises first and second terminals, the first terminal being coupled to a voltage supply, the second terminal being coupled to the first output terminal; andwherein the second resonant tunneling diode comprises first and second terminals, the first terminal being coupled to the voltage supply, the second terminal being coupled to the second output terminal.
 16. The signal level determination circuit of claim 10, wherein the comparator further comprises:a first current source coupled to the first output terminal operable to supply a first offset current to the first output terminal, the first offset current offsetting at least a portion of the first current; and a second current source coupled to the second output terminal operable to supply a second offset current to the second output terminal, the second offset current offsetting at least a portion of the second current.
 17. The signal level determination circuit of claim 13, wherein the current sink comprises:a first current sink coupled to the second terminal of the first transistor; a second current sink coupled to the second terminal of the second transistor; and an impedance connected between the second terminal of the first transistor and the second terminal of the second transistor. 